Carbon-ceramic brake discs consist of a carbon-fibre-reinforced ceramic material whose matrix contains silicon carbide. They can be produced by infiltrating a carbon-fibre-reinforced porous carbon body with liquid or gaseous silicon, at least part of the carbon in the matrix reacting with the silicon to form silicon carbide. Depending on the amount of silicon used for infiltration, a larger or smaller part of the pores remains in the material produced. In addition, process-related microcracks can form in the cooling phase after siliconisation due to the differing coefficients of thermal expansion of the phase components.
At least some of the remaining pores are accessible from the surface of the carbon-ceramic brake disc. Due to absorption of moisture from the environment (air or water when driving on wet roads), these pores can contain water, which evaporates when the temperature of the disc rises on braking and escapes from the pores as gas. The escaping gas can form a lubricating film between the brake disc and the brake linings and thus delay the brake response. Whilst in the conventional combination of cast iron brake discs with organically bonded brake linings this phenomenon is limited to the film of moisture adhering to the surface of the disc and the porosity of the brake linings and the gases escaping from them, due to the porosity of the ceramic brake disc materials the amount of escaping gases, and hence the time before the gas layer between brake and lining collapses, can be increased markedly. This behaviour has also been verified by experiments, wherein on wet braking the time until the brake response and the start of build-up of the coefficient of friction between disk and lining is longer in the case of the carbon-ceramic brake disc than in the case of the combination of a grey cast iron brake disc with a conventional brake lining. In the case of the carbon-ceramic brake disc, however, the build-up of the coefficient of friction is significantly faster, so the disadvantage in the response behaviour with the carbon-ceramic brake disc is more than compensated for by the fact that the dry coefficient of friction is reached more quickly.
It is desirable, however, to reduce or improve this delay in the onset of the braking action in comparison to the combination of a grey cast iron brake disc with a conventional lining.
It has further been observed that on exposure to salt solutions, particularly those arising from the thawing of mixtures of snow or ice with de-icing salts used in winter, firstly the phenomena in the wet braking behaviour described here are intensified and secondly, on frequent and relatively extended exposure to such salt solutions, chipping can occur on the friction surface in carbon-ceramic brake discs.
Attempts to reduce the porosity of carbon-ceramic brake discs by infiltration with other liquid metals or with liquid inorganic materials which harden to form a glassy substance, for example, have not achieved their aim.